1. Field of the Invention
The present invention relates to a disc-shaped recording medium which has a plurality of tracks each having header fields at which address information is pre-formatted in the form of embossed pits.
2. Description of Related Art
The CLV (Constant Linear Velocity) system which keeps linear velocity of an optical disc constant and the CAV (Constant Angular Velocity) system which keeps angular velocity of an optical disc constant are employed as recording and/or reproducing systems for optical discs. Both of the systems have defects. That is, the CLV system is inferior in ability for having accesses, while the CAV system has low recording density at outer circumferences. So, in order to compensate for these defects, the ZCAV system and ZCLV system are employed. In both systems, recording/reproducing area of an optical disc having a plurality of tracks is divided into a plurality of groups (zones) along the radial direction, and the CAV system is employed within each of the zones. In the ZCAV system, recording frequency is changed in accordance with linear velocity without changing angular velocity when a pick up transfers from one zone to another. On the other hand, in the ZCLV system, angular velocity is changed without changing recording frequency when a pick up transfers from one zone to another.
In the ZCAV system and ZCLV system, the recording medium itself has the same configuration. As described above, the recording/reproducing area of the optical disc having a plurality of tracks is divided into a plurality of zones from the innermost circumference to the outermost circumference, and the CAV system is employed in recording/reproducing data within each of the zones. Furthermore, each of the tracks within a zone has the same number of pre-formatted header fields in one circumference, and a data recording field of a predetermined length is provided following each of the header fields. From the beginning of a header field to the end of a data recording field or before the next header field is termed a sector. The number of sectors within a track of an outer circumference is larger than that within a track of an inner circumference.
Since the number of header fields within a track in an outer circumference zone is larger than that within a track in an inner circumference zone, the positions of header fields within a track in a zone do not accord with those of header fields within a track in an adjacent zone along the track direction at borders of zones. That is, there are caused gaps between the positions of header fields within a track in a zone and those of header fields within a track in an adjacent zone at borders of zones. In case the positions of header fields of adjacent tracks do not accord with each other, there arise various problems. As shown in FIG. 1, since a header field 131 of an inner zone 130n adjoins a data field 132 of an outer zone 130n+1, the influence of signals of the header field 131 appears as crosstalk at the data field 132. Accordingly, data recorded in at least one track cannot be reproduced because of the influence of crosstalk.
Since resin spreads out from the inner side to the outer side when an optical disc is molded, there arises a variation of refractive index or double refraction at the data field 132 which adjoins the header field 131 along the radius direction, which deteriorates signal quality of data fields 133.
Thus, data cannot be correctly reproduced at tracks including the data fields 133 due to the influence of crosstalk or double refraction.
For example, in case the data fields 133 which are subject to crosstalk or double refraction are included in four tracks, four tracks in the zone 130n and four tracks in the zone 130n+1 cannot be used for recording/reproducing data, as shown in FIG. 1. Thus, the total number of eight tracks cannot be used for recording/reproducing data.
There is disclosed a technique concerning an optical information recording medium which has tracks consisting of only one or plural guide grooves at the borders of zones in the Japanese Laying-Open Patent H 4-315823. FIG. 2 shows an enlarged view of adjacent zones of an optical disc disclosed in the Japanese Laying-Open Patent. As shown in FIG. 2, the optical information recording medium has border area 141 consisting of, for example, four tracks of guide grooves between usable tracks in a zone 140n, and usable tracks in a zone 140n+1. Thus, data recorded in data recording fields of the usable tracks are not influenced by crosstalk or double refraction, and there exists no data recording fields whose data cannot be correctly reproduced. For this reason, since the optical information recording medium has border area 141 consisting of only four guide grooves at the borders of zones, the total number of unusable tracks at a border between two zones which cannot be used for recording/reproducing data becomes four or half that of the optical disc shown in FIG. 1 which has eight unusable tracks at a border between two zones.
However, in case the optical information recording medium has the border area 141 consisting of only guide grooves at the borders of zones, if a pick up transfers to a track in the border area 141 at the time of having an access to the zone 140n which is adjacent to the border area 141, the pick up cannot find information to transfer to a target track since the tracks in the border area 141 has no header information. Thus, the pick up becomes uncontrollable.
It is therefore an object of the present invention to overcome the above-mentioned drawbacks by providing a disc-shaped recording medium whose tracks at borders of zones have header information to enable a pick up to transfer to a target track, and whose data fields are not influenced by crosstalk or double refraction caused by the header fields.
According to the present invention, there is provided a disc-shaped recording medium having formed thereon a plurality of tracks each composed of a plurality of sectors, each of the sectors having a header field at which address information is recorded in the form of embossed pits and a data recording field provided following the header field,
wherein the tracks are divided into a plurality of groups along the radial direction, and
wherein, in two groups adjacent to each other via a border area formed between them, L being the number of the header fields included in a track of the inner group, M being the number of the header fields included in a track of the outer group, and N being the number of the header fields included in a track of the border area satisfy the following equation,
1xe2x89xa6N less than L, and 1xe2x89xa6N less than M, and 
wherein the header field included in at least a track of the border area adjoins the header field included in a track of the inner group and the header field included in a track of the outer group along the radial direction with a gap of a predetermined length along the track direction within a range in which data recorded on the data recording field is not influenced by double refraction caused by the header field.
According to the disc-shaped recording medium of the present invention, tracks in border areas of adjacent zones have header fields which adjoin those of the adjacent zones. Thus, each of the tracks in the border areas has at least one header field, and having accesses to the disc around border areas of adjacent zones becomes possible, and data recorded in data recording fields is not influenced by crosstalk or double refraction caused by the header fields.